FWIW. cj, sorry, but these regulators require at least 2.5 volts or a little more ABOVE the output voltage, to 'safely regulate', (or switch on, so to speak). So By using a 12 volt reg. to regulate a 12 volt battery will not work. The max applied voltage is 35 volts. HTH..
L.
ps. Paralleling plug packs is not advisable either due to 'not sharing the load evenly'. One will supply more than the other & o/load...eventually taking the other with it...